FIG. 1 schematically illustrates an example in which a coating solution is spin-coated on a substrate for coating, using a conventional method. A conventional spin coating method is performed as follows. A liquid coating solution is dropped on the central portion of the substrate for coating which is rotating at a low speed, and then the substrate is rotated at a high speed. Then, the coating solution spreads outward from the center of the substrate due to the centrifugal force and is then applied to the entire substrate.
However, where a high-viscosity coating solution is applied to the substrate using a conventional spin coating method, viscoelasticity and surface tension of the coating solution result in a conglomeration portion of the coating solution at end portions of the substrate (see 20a of FIG. 1), which in turn, upon curing in such a state, forms a hill, thereby resulting in a ski-jump. Such a ski-jump leads to difficulty in formation of uniform coating throughout the entire substrate.
As the viscosity of the coating solution is higher, the ski-jump phenomenon is more pronounced. In manufacturing a variety of materials such as optical discs, semiconductor substrates, acoustic matching layers and the like, the high-viscosity coating solution is usually spin-coated on the substrate, and it is impossible to obtain a uniformly coated film, due to such a ski-jump. Hereinafter, problems associated with the ski-jump, exhibited when preparing several conventional substrates for coating, will be described, although application of the present invention is not limited thereto and the present invention can be applied to any fields suffering from the ski-jump problems occurred in preparation processes to which spin coating is applied.
1. Optical Disc
Generally, optical discs are widely used as information-recording media for use with an optical pickup which records/reproduces information in a non-contact manner. With changes in their recording capacity from compact discs (CDs) with a recording capacity of 600 to 800 MB to digital versatile discs (DVDs) with a recording capacity of 4 to 10 GB, optical discs have been developed toward improvement in a degree of integration of data. Recently, in order to offer high tone quality and high picture quality, a Blu-ray disc (BD) has been developed which is an optical disc having a recording capacity of more than 20 GB and using a blue laser to access the information. According to the System Description Blu-ray Disc Rewritable Format, a read-out area where information on the substrate is recorded is defined with a radius of up to 58.5 mm. Therefore, a data-recording area must secure a minimum radius of 58.5 mm, i.e. a diameter of up to 117 mm. For example, a 100 μm-thick cover layer should have a uniform thickness of 100±2 μm over a radius of 58.5 mm, and the remaining peripheral area of 1.5 mm should have a ski-jump of less than 10 μm in height. However, when a UV-curable resin layer is coated on the optical disc via the conventional spin coating method, a resin layer having a thickness of 100 μm exhibited a ski-jump of more than about 45 μm, which was very thick, thus failing to satisfy the above-mentioned specification.
In order to solve problems associated with the ski-jump, Korean Patent Publication Laid-open No. 2003-0004541 discloses a method of manufacturing for optical disc involving preparing a substrate having a larger diameter than an optical disc that will be finally completed, applying a UV-curable resin to the substrate, thereby forming a cover layer having a predetermined thickness, irradiating UV light to a bump lifted up at the peripheral portion of the cover layer so as to cure the resin and cutting the cured bump. However, this method requires an additional step to remove the bump in an optical disc production process, which in turn further increases manufacturing facility costs and manufacturing process time, thereby lowering manufacturing efficiency, and suffers from problems such as susceptibility to fragility, cracking and bending of the substrate upon cutting the bump.
Korean Patent Publication Laid-open No. 2001-0055044, as shown in FIG. 2, discloses a spin coating method of optical disc involving mounting a disc in a disc housing (see numerical reference 30 of FIG. 2) having the same inner diameter as an outer diameter of the disc and having the same depth of a disc-mounting groove as the thickness of the disc and spin coating UV-curable resin thereon. However, in this case, there are disadvantages such as difficulty to take the disc out of the disc housing after spin coating, contamination risk of a rear side of the disc due to inflow of the resin between the disc and disc housing and then along the side of the disc, and severe contamination of the optical disc due to accumulation of the coating solution in a jig upon repetitive coatings and difficulty to drain it. Further, although there is reduction in the size of the ski-jump, as compared to the conventional spin coating method, it is still difficult to obtain a uniform resin layer to the extent of a disc radius of 58.5 mm and there is a formation of the ski-jump of more than 20 μm thickness.
2. Integrated Circuit (IC)
In formation of an integrated circuit on a semiconductor wafer, in order to obtain any elements via selective implantation of impurities into a predetermined region on the semiconductor wafer or formation of a thin film layer thereon, a photolithography process using a photoresist film is usually employed. The photolithography process is carried out by coating the photoresist film to a thin thickness on the semiconductor wafer, exposing the coated photoresist film using a mask and developing it. This is followed by implantation of impurities through an opening of the photoresist film, or removing the photoresist film after formation of a thin film layer. Formation of the photoresist film is usually carried out using a spin coating method including applying a predetermined amount of a photoresist to the central part of the top surface of the semiconductor wafer and rotating the semiconductor wafer, thereby forming a film of the applied photoresist to a uniform thickness on the entire upper surface of the semiconductor wafer. In this case, upon implantation of impurities or formation of the thin film layer, it is not easy to form a photoresist film to a thick thickness of several tens of μm. This is because the photoresist itself, forming the photoresist film, has a predetermined viscosity and therefore a thick photoresist film is formed at edges of the semiconductor wafer upon performing spin coating. In fact, when the photoresist film having a thickness of 60 μm is coated on the semiconductor wafer via spin coating, the ski-jump having a width of about 7 mm and a thickness of 120 μm is occurred inward from edges of the semiconductor wafer.
In order to overcome these problems, Korean Patent Publication Laid-open No. 2001-0017145 discloses a method for forming thick photoresist film involving forming a first photoresist film on the top surface of the semiconductor wafer to a thickness ½ times that of photoresist film to be formed, via coating and soft curing processes, removing edge beads (ski-jump) of the first photoresist film formed at edge portions of the semiconductor wafer using a thinner, and finally forming a second photoresist film via coating and soft curing processes, thereby forming a photoresist film having a desired thickness. This method requires performing both a coating process and a curing process twice, thus leading to a complicated process and increased process time, and also presents a problem associated with additional costs for treatment of waste liquor generated due to use of the thinner to remove edge beads of the first photoresist film.
3. Multi-Layer Interconnection Circuit
Various kinds of integrated circuits (ICs) are used in many different electronic instruments such as computers. In addition, with a trend toward miniaturization and high performance of electronic instruments, there is a need for improvement in manufacturing reliability requiring high precision and high performance of circuits. As such, in order to increase a degree of integration of conventional semiconductor ICs, a multi-layer interconnection circuit, as shown in FIG. 3, is employed. Briefly describing a manufacturing process of such a semiconductor substrate, a first insulating film 32 and an oxide film are formed on a substrate 31 such as silicon, and a first interconnection layer 33 made up of an aluminum film or the like is formed on the surface of the first insulating film 32. Next, an interlayer dielectric (ILD) 34 such as a silica film or a silicon nitride film is applied thereon via chemical vapor deposition (CVD) or plasma enhanced chemical vapor deposition (PECVD). For planarization, a silica insulating film (a planarization film) 35 is formed on the interlayer dielectric (ILD) 34, and if necessary, a second insulating film 36 is then formed thereon. Next, a second insulating layer (not shown) is formed and if necessary, a second interconnection layer, interlayer dielectric, planarization film and insulating film may be formed. Such a multi-layer interconnection structure is used as a highly integrated circuit.
Examples of a method for forming the interlayer dielectric (ILD) include the following methods: 1) a method of forming a SiO2 film on the surface of the substrate via chemical vapor deposition (CVD) using chemical gas such as SiH4, 2) a method of forming a SiO2 film via plasma-deposition of tetraethoxysilane (TEOS), and 3) a method of forming a SiO2 film by applying a coating solution for formation of a silane-based insulating film, which is a coating solution for formation of a siloxane-based insulating film, to the surface of the substrate, via spin coating. Among these methods, the third method for forming the interlayer dielectric is preferred as a method having high processing capability and capability to form a planar film. However, upon spin coating the coating solution, it is disadvantageous in that convex portions (ski-jump), as shown in FIG. 1, are formed at a periphery of a silicon wafer. Thus, the periphery of the silicon wafer contacts with other structural elements and are contaminated, or when conveying the silicon wafer, convex portions at end parts around the insulating film contact with other devices, thereby creating cracks. When cranks occur, a large amount of foreign materials occur, thereby significantly decreasing productivity.
In order to solve such problems, Japanese Patent Publication Laid-open No. 1996-316186 discloses a method involving cleaning and removing convex portions formed around the silicon wafer by discharging a solvent downward onto the convex portions after spin coating. In this method, different cleaning solvents should be used depending upon kinds of an insulating film-forming coating solution (kinds, concentration and solvents of insulating components), and additional bumps (protrusions) may be formed during cleaning and removing processes. Such bumps may be a cause of foreign material such as convex portions.
4. Ultrasonic Transducer
An ultrasonic endoscope provides an ultrasonic tomogram by scanning oscillating ultrasonic beams, generated from an ultrasonic transducer, along a predetermined path, receiving again ultrasonic waves reflected from inner walls of the internal organs and pathogenic lesions through the ultrasonic transducer and preparing an image based on the thus obtained information. The ultrasonic transducer is generally composed of piezoelectric ceramics. There is a large difference in acoustic impedance between the piezoelectric ceramics and living body and therefore reflection and loss of ultrasonic waves are occurred at interface therebetween. In order to absorb such a difference and decrease acoustic loss, an acoustic matching layer is installed at an acoustic emission side of the piezoelectric ceramics. Since the ultrasonic transducer for the ultrasonic endoscope has an oscillation frequency ranging from several MHz to several tens of MHZ, the acoustic matching layer, which generally has a thickness ¼ times that of an ultrasonic wavelength, is fabricated to have a thickness of several tens of μm such that the speed of sound in a conventional resin is 2500 to 3000 m/s. When the acoustic matching layer is formed by the spin coating method, the viscosity of the resin should be significantly high and thus thickness-unstable portions (ski-jump) are occurred around the acoustic matching layer.
In order to solve such problems, Japanese Patent Publication Laid-open No. 1993-103396 proposes a method of manufacturing an acoustic matching layer involving dropping a UV-curable resin on a substrate, rotating the substrate to diffuse the UV-curable resins irradiating UV light to cure the resin under steady state at which centrifugal force exerted by rotation of the substrate and physical properties such as viscosity and surface tension of the resin are equilibrated, and cutting and removing the thickness-unstable portions. In this method, as UV light is irradiated from a spin coater, the UV-curable resin remaining on the spin coater is simultaneously cured, thus making it difficult to remove the remaining resin, and an additional step to cut the thickness-unstable portions is necessary, thus resulting in decreased productivity.
As such, a spin coating technique is widely used in various fields, but suffers from fundamental problems associated with formation of a ski-jump. Therefore, there is a need for the development of a method capable of solving problems of the ski-jump without causing contamination of the substrate or decreasing working efficiency during a manufacturing process of substrates for coating.